Medical ultrasound scanners typically produce a two-dimensional grey scale image of a planar region of a subject's body. The grey scale image is created by transmitting a series of ultrasound pulses into the region under investigation, and receiving and processing the echoes of the transmitted pulses, to build up a two-dimensional image. Modern ultrasound scanners are capable of obtaining images at a fast enough rate, e.g., twenty or more times per second, so that a real time display of the region under investigation can be created on a video monitor.
Modern ultrasound systems generally include the capability of obtaining Doppler data from a selected volume within the two-dimensional region under investigation. The Doppler data represents the velocity of structures within the selected volume. Thus if the selected volume is within or includes a blood vessel, important information can be obtained concerning blood velocity. Such information is extremely useful in diagnosing various cardiac and other circulatory problems.
A relatively recent innovation in ultrasound systems is the use of "color flow" systems to combine a grey scale image and Doppler data. In a color flow system, all or a selected portion of a grey scale image is overlaid with a color image, with different colors corresponding to different velocities. Color flow systems must obtain Doppler data from a large number of sample volumes throughout a portion of an image. Since several pulses are required for each sample volume, the frame rate that can be obtained in color flow systems tends to be quite low. A low frame rate makes the display jerky, and can cause events of short time durations, such as small flow disturbances and valve leaks, to be missed entirely. There is therefore a substantial need for ultrasonic systems, particularly color flow systems, that are capable of producing frame rates high enough to provide real time imaging.
In a number of medical imaging systems, including CAT scans, MRI, PET, and thallium scanning with a gamma camera, it is known that imaging can sometimes be facilitated by taking advantage of the fact that the motion of a portion of the heart or circulatory system of a subject is substantially duplicated from one heartbeat to the next. Thus a number of such imaging techniques provide the ability to acquire a series of images or frames at fixed time delays after the occurrence of an R wave in an ECG signal. The acquisition is repeated for a number of subsequent cardiac cycles, and the results are then averaged and displayed. These techniques utilize a long integration time to produce a usable image or image set. However they have the effect of increasing acquisition time to produce a set of images at reasonable frame rates, rather than producing high frame rates in real time.